Most Western countries have developed guidelines and programs to inform and promote regular physical activity and exercise behaviours in order to gain desired health benefits for the population. Unfortunately, health statistics show that many people do not reach these recommended activity levels. Research has also demonstrated that approximately half of those who actually try fail to maintain regular exercise habits. Theoretical understanding of the mechanisms of motivation is of great importance for how to enhance the knowledge of how interventions promoting sustainable exercise motivation and behaviour can be designed. The overall aim of this thesis was to explore the motivational processes behind physical activity and exercise behaviours, with the self-determination theory as a guiding framework. Previous research and practice have generated ample knowledge of what works in exercise and physical activity promotion on a general level, but less is known about why it works, that is, the underlying mechanisms. Because interventions operate through mediating processes, the study of indirect effects and motivational mechanisms may forward mean level research and has the potential to provide knowledge of how observed intervention effects could be interpreted and understood. A key finding of this thesis was that analyses of Study I and IV showed patterns of need satisfaction, motivational regulations, and exercise differing across age and gender, indicating that motivational mechanisms could vary (qualitatively) in different subgroups. These findings support the idea that a generic method will not be successful in all situations and for all participants (i.e., one size does not fit all). Based on the results of Study II and IV, a second key finding is that the mediating mechanisms of the process model can be manipulated in an intervention by, for example, creating need-supportive environments facilitating internalization and subsequent exercise behaviour. In line with previous research, both Study I and II demonstrated identified regulation as playing a prominent role in the motivational processes, supporting the significance of internalizing the values behind a certain behaviour for the regulation of potentially challenging activities such as exercise. This is also why you don’t have to love it as long as it suits your life routines and feels valuable to you. A third key finding is related to the findings of Study III, which provide preliminary support for the notions behind “motivational soup” by showing motivational profiles based on person-centred analyses. Finally, in Study IV, amotivation was involved in significant main (time) effects and also played an unexpected role in the motivational processes of younger adults.... more